The present invention relates to a switched-capacitor filter having, at its input, a signal converter for creating so-called 100% sample and hold signals to be supplied to the switched-capacitor filter.
Stimulated by the progress in MOS (metal oxide semiconductor) technology, considerable interest has recently arisen in the possibility of filters realized by means of switched-capacitors, that is, in the switched-capacitor filter (hereinafter often referred to by the abbreviation "SCF"). The major advantages of the SCF reside in the facts that only capacitors, operational amplifiers, and switches are needed, that nearly perfect switches can easily be built, and, especially, that all resonant frequencies are determined exclusively by capacitance ratios. Therefore, the SCF is very useful in various kinds of electronic processing systems, for example, in PCM (pulse code modulation) communication systems. In a PCM communication system, as will be explained hereinafter, a low-pass filter is one of the important electronic processing members. The low-pass filter operates in such a manner as to transfer only signals having frequencies lower than 4 KHz and, accordingly, only voice signals can pass therethrough. In a PCM communication system, it is preferable to build the low-pass filter by means of an SCF.
Although the SCF itself has the above mentioned advantages, it must cooperate with the signal converter to create so-called 100% sample and hold signals, when it is utilized as the low-pass filter in the PCM communication system, especially in a receiver station thereof. Such an SCF usually receives successive sample and hold PAM (pulse amplitude modulation) input signals which are supplied from a digital/analogue converter, i.e. a decoder. The decoder receives the PCM input signal, transmitted via a transmission line, from a transmitter of the PCM communication system. Generally, since the decoder is basically comprised of a sample and hold circuit, the PAM input signal to be supplied to the SCF has a particular constant holding term, which is determined in accordance with the respective sample and hold circuits. In this case, it is a well known fact that undesired distortion frequency characteristics of the output signal from the SCF is produced during application of said PAM input signal to the SCF, which varies proportionately with the holding term due to the so-called aperture effect. Therefore, when the holding term of the PAM input signal varies, the output signal from the SCF also varies due to a variation of the above-mentioned distortion of the PAM input signal. Consequently, there is a shortcoming in that it is difficult to obtain, from the SCF, output signals having the desired filtering characteristic without error.
In order to eliminate the above mentioned shortcoming from the decoder, a signal converter for creating 100% sample and hold PAM input signals, has already been proposed in, for example: INTEL LTD. 2910 PCM CODEC-.mu.LAW, 8-BIT COMPANDED A/D AND D/A CONVERTER; and ISSCC 80/THURSDAY, Feb. 14, 1980/IMPERIAL BALLROOM/3:45 P.M. SESSION XIV: TELECOMMUNICATION CIRCUITS. The proposed signal converter is mainly comprised of a switch, a capacitor, and an operational amplifier, and is located between decoder and the SCF (which will be explained hereinafter). This signal converter is a sample and hold circuit, for so obtaining 100% sample and hold PAM input signals so that a non-holding term is not included therein.
However, the previously proposed signal converter produces the following two defects. One of the defects it that it is difficult to incorporate the signal converter in the SCF on an IC (integrated circuit) as one body. Therefore, the chip size may become large. This is because the capacitor of the proposed signal converter must have a very large capacitance value, for example on the order of several hundreds of pF.
The other defect is that the power consumption of the previously proposed converter is relatively large. This is because, as previously mentioned, the signal converter includes an operational amplifier which requires energizing power.